This invention is related to wireless networks, and in particular to a wireless station that includes more than one receive antenna, and to a method of selecting the receive antenna to use according to a measure of the signal quality. In one version, the measure of the signal quality is a measure of the relative error vector magnitude (EVM).
Wireless networks such as wireless local area networks (WLANs) have recently become popular. A WLAN may be an ad-hoc network in which any wireless station (STA) may communicate directly with any other STA, or an infrastructure network in which one STA acts as an access point (AP).
The description herein will assume a wireless network that conforms to the IEEE 802.11 standard, and will use the terminology of the IEEE 802.11 standard. In particular, the invention will be described with reference to variants of the IEEE 802.11 standard that use orthogonal frequency division multiplexing (OFDM) wherein a signal is transmitted as a set of subcarriers. Such variants include IEEE 802.11a and 802.11g. The invention, however, is not restricted to such a network. For simplicity, 802.11a will be used to refer to any of the OFDM variants of the IEEE 802.11 standard.
Multipath refers to multiple transmission paths between transmit and receive antennas of stations, and causes both frequency-selective fading and space-selective fading. Frequency-selective fading means that the channel varies with frequency. Space-selective fading means that the channel is dependant upon the position of the transmit and receive antennas. FIG. 1 shows the subcarrier powers (as 802.11a channel estimates) observed for the same packet received through two vertically oriented dipole antennas separated by a half-wavelength (λ/2) in an office environment and demonstrates the existence of both frequency-selective fading and space-selective fading. In FIG. 1, the total channel 1 power is −57.2 dBm, and the total channel 2 power is −60.3 dBm. The frequency-selective fading is evident from the fact that there is significant subcarrier power variation in both channels. The space-selective fading is evident from the fact that the two channels look completely different with only a λ/2 spacing between the two antennas.
As with many other digital wireless network protocols IEEE 802.11a uses forward error correction (FEC) to add redundancy to the transmitted data so that a receiver can recover the transmitted data even if certain data bits are corrupt. Combining OFDM and FEC gives 802.11a receivers the ability to recover a transmitted packet even if certain subcarriers within the packet are not recoverable. This is especially important in a multipath environment where frequency-selective fading can result in more than 30 dB of subcarrier power variation within a packet. There are frequently situations, however, that the multipath fading is too severe for 802.11a even with its inherent ability to deal with multipath. In these situations, it is necessary to implement a technique that mitigates the effects of the multipath to ensure a reliable link.
One approach that has commonly been used in wireless communications is to take advantage of the space-selective fading by using multiple receive antennas separated by a sufficient distance.
Several approaches are now discussed.
A first prior art method that uses spatial diversity with the two receive antennas includes a separate receiver connected to each receive antenna. Such a system is shown in simple form in FIG. 2. A pair of antennas 201, 202 is coupled to respective receivers 203 and 205 that fully demodulate the signals received at each antenna. The method includes fully demodulating the signal received on each antenna. The antenna selection circuit 209 accepts the demodulated output and provides control to an antenna switch 207 to selecting the data set with the least error.
A second prior art method is shown in FIG. 3. A pair of antennas 301, 302 is coupled to respective receivers 303 and 305, so this method also includes a receiver for each antenna. A combiner 307 combines signals produced by the receivers. The second method includes combining the two signals, e.g., using maximum ratio combining or some other forms of combining, to provide a composite signal with better signal quality than either signal alone. That composite signal is then demodulated and further processed.
Many configurations are possible. In the OFDM version shown, the receivers 303 and 305 each include an analog to digital converter to produce samples signals, an FFT processor (not shown) and a demodulator to produce symbols. The receivers 303 and 305 also each produce estimates of the channel. The combiner 307, in this case a maximum ratio combiner, uses the channel estimates and the demodulated symbols to produce combined demodulated symbols. An OFDM signal processing circuit 309 converts the symbols to bits, de-interleaves, performs any necessary depuncturing, Viterbi decodes, de-scrambles, and performs a CRC check as is known in the art.
The disadvantage of these two prior art methods, however, is that they both require a receiver per receive antenna.
A much more economical approach is to have a single receiver that can alternately connect to each of the two antennas during the start of the packet and select the antenna based on some decision metric. This approach is referred to as selection diversity, and is the subject of the present invention. Selection diversity is commonly used in 802.11a stations.
One prior art decision metric is based entirely on the strength of the received signal, as indicated by the received signal strength indication (RSSI). FIG. 4 shows a conceptual design of such a receiver with two antennas 401, 402. The control circuit 403 provides a control to a switch 405 to select the signal from one or the other antenna based on signal power. The selected output of the switch is accepted by a single receiver 407.
The IEEE 802.11a physical layer (PHY) standard defines RSSI as a measure by the PHY sublayer of the energy observed at the antenna used to receive the packet. RSSI is measured by the PHY during packet reception and is passed up with the packet.
FIG. 5 shows a more practical version of a system that selects the antenna according to the RSSI. Modern receivers for WLANs typically produce the RSSI. In FIG. 5, the signals from two antennas 501, 502 are fed to a switch 503 that selects one or the other antenna for a radio receiver 505. The radio receiver produces an RSSI output. The RSSI output is fed to an antenna controller 507 that controls the switch 503. Initially, one or the other antenna is selected. After a start-of-packet is detected, the controller 507 selects one then the other antenna and compares the RSSI that results from each antenna. These RSSI values are used to differentiate the signal strength from the candidate antennas and to determine the “best” antenna according to the received signal strength.
Those in the art will recognize that the RSSI is a measure of signal strength but not signal quality. We have found that indeed the RSSI is not a good indicator of the signal quality or in itself a good measure for “best” antenna selection. An RSSI value, for example, does not account for factors that significantly reduce signal quality such as multipath.
Using the RSSI to select the “best” antenna can result in a lower throughput and latency than would happen if a measure of signal quality rather than signal strength was used for the selection.
Thus there is a need in the art for a method of selecting an antenna in a receiver that includes multiple antennas using a metric such as a signal quality measure that takes into account frequency selective fading. There also is a need in the art for a receiving method that uses multiple antennas but that does not require a receiver per receive antenna.